This invention relates to a process for the dry distillation of rubber blocks, obtained from used tires, rubber scrap and the like which are industrial waste materials, in a heating oven such as a fluidized bed-forming oven. More particularly it relates to a process for dry distilling such rubber blocks by heating the blocks for the pulverization thereof and then heating the pulverized blocks for the pyrolysis thereof in such a heating oven or by heating the blocks for the substantially simultaneous pulverization and pyrolysis thereof in the heating oven.
With the recent remarkable progress of industry and economy, the amount of rubber articles demanded has rapidly increased with the result that large quantities of used rubber articles and rubber scrap accompanied with the preparation of rubber articles have been produced as industrial waste or refuse. Thus, various studies have been made in an attempt to find new uses of such large quantities of rubber waste in addition to conventional uses thereof.
The primary object of this invention is to provide a new process for dry distilling the rubber waste to convert the same to oily materials and residues which are both useful materials, in contrast to conventional processes for disposing of such rubber waste by burning.
This object is achieved by mixing a powdery or granular thermal medium with the rubber waste in block form such as used tires, rubber scrap, synthetic rubber rejects from synthetic rubber processing steps, and the like which should be rejected; heating the resulting mixture to 250.degree.-400.degree. C. under agitation to make the rubber blocks brittle and to pulverize them and further heating the thus-obtained rubber powder to 400.degree.-500.degree. C. while floating and flowing the powder in a stream of air or any other oxygen-containing gas, that is, forming a fluidized bed of the powder, to thermally decompose the powder into oily and solid components which are then separately recovered with a high yield.
The aforementioned object is also achieved by heating the rubber blocks to 350.degree.-800.degree. C. under agitation while producing carbon powder as a dry distillation residue that serves as a powdery thermal medium to pulverize the rubber blocks into rubber powder and substantially simultaneously pyrolyze the powder while floating and fluidizing the powder in an oxygen-containing gas stream and burning part of the powder to supply the necessary heat to the system, thereby obtaining oily materials and solid residues (carbonized materials).
In pulverizing blocks of used rubber, a mechanical pulverizing method has heretofore been employed. To practice the method, however, requires a very secure apparatus, and the apparatus needs the removal of metallic scrap, such as iron scraps, possibly present in the rubber blocks before the apparatus is used. Such apparatuses are very noisy during their operation and they are also disadvantageous in that they cannot pulverize all of the rubber varying from hard vulcanized rubber such as tires to very soft unvulcanized ones.
In contrast, according to this invention, the rubber blocks are pulverized by chemical and physical actions caused therein by heating, and the blocks so pulverized is pyrolyzed subsequent to, or substantially simultaneously with, the pulverization under approximately the same operational conditions as the pulverization, thereby allowing said two treatments to be effected substantially one after another or substantially simultaneously with each other in the same apparatus.
When the rubber blocks are heated to high temperatures, (1) the molecules in the outer layer of the rubber blocks are cross-linked with each other through the double bonds present in the blocks or radicals produced by the severance of the molecules, the liberation of the side chain groups, or the like. When such crosslinking proceeds, the outer layer of the rubber blocks will be in a suitably hardened state as in thermoset plastics and will, in certain cases, come to crack while the inner layer thereof will be in a molten state. The rubber blocks in such state tend to cause chipping at the partial portions of the outer layer thereof and further cause peeling of the outer layer from the inner layer by applying a relatively small stress to the blocks. Thus, when the rubber blocks are in said state under agitation, they will cause gradual peeling of their outer layer therefrom by the force of collision and friction between the rubber blocks themselves, between the blocks and the agitator, and between the blocks and said solid particles. The agitator may stop its rotation for several seconds due to a heavy load just after the charge of the rubber blocks, after which it restarts its rotation. In addition to such phenomena as mentioned above, (2) the rubber blocks in said state not only melt but also cause decomposition and polymerization of the molecules in the melted inner layer thereof to produce therein gaseous low-molecular compounds (hereinafter sometimes referred to simply as "volatile components") having a gas pressure by which the blocks are ruptured and disintegrated, the rupture and disintegration starting at the inner layer. At this time the outer layer is finely divided or separated from the inner layer in such a manner that it flies off in pieces. The rubber blocks are thus pulverized.
In the outer layer of the rubber blocks when initially exposed to high temperatures and the inner layer exposed after the peeling-off of the outer layer as in the case (1) mentioned above and in the inner layer exposed after the disintegration of the blocks or the peeling-off of the outer layer as shown in the case (2), the rubber portions of the blocks are melted and viscous and they therefore adhere to each other or to the apparatus in which the blocks are treated. Such adhesion of the rubber portions when heated, can be prevented by mixing them with, for example, gravel, glass baloons, i.e., hollow glass globules, rubble, powdered carbon, powdered iron, carbonized rubber or carbonized synthetic resins, coke, magnetite, terra alba or any other solid particles capable of preventing the melt adhesion of rubber when heated (these solid particles being hereinafter sometimes referred to simply as "melt adhesion-preventing solid particles"). It is a matter of course that these particles may also serve as thermal media.
As is clear from the above description of the pulverization of the rubber blocks, the expression used herein that "the rubber blocks are made brittle according to this invention" is intended to mean that the rubber blocks are made brittle by heat treatment to the extent that they can be pulverized as indicated in the cases (1) and (2).
According to this invention, as previously mentioned, in obtaining the powdered rubber easily from the rubber blocks, advantage is taken not only of the chemical actions such as the inter-molecular crosslinking reaction of the rubber blocks as well as the decomposition and polymerization thereof but also of the physical actions such as the friction and shock produced by the relatively small stress as well as the gas pressure exerted by the volatile components produced by the decomposition and polymerization of the rubber blocks.
The ovens which may be used in the pulverization of the rubber blocks according to this invention include fixed bed-forming ovens, fluidized bed-forming ovens and any other suitable ovens, and the agitating devices which may be used herein include vane-type stirrers, gas stream-agitating devices and any other suitable devices. For this agitation purpose, rotary ovens may also be used. It would be possible for those skilled in the art to select a suitable one of said ovens or devices depending on the purpose for which it is used. The powdered rubber obtained may be recovered by filtration, separation based on difference in specific gravity, or the like.
In the step of pulverizing the rubber blocks according to this invention, they are usually heated to temperature in the range of 300.degree.-390.degree. C. These temperatures vary depending on such factors as the kind and properties of rubber blocks used and the desired particle size of powdered rubber to be obtained; however, a suitable temperature to be used may easily be selected from those in said range by making preliminary tests for determining the suitable temperature. For example, a temperature of 250.degree. C. is a satisfactory one at which the powdered rubber can be obtained in cases where an oxygen-rich gas is used in the system. However, there should be avoided the use of high temperatures at which the powdered rubber is violently pyrolyzed. The particle size of powdered rubber to be obtained may be controlled or adjusted by the selection of pulverizing conditions such as heating temperature, heating time, agitation stress, the kind, size and amount of melt adhesion-preventing solid particles and the amount of gas introduced to a fluidized bed-forming oven which is the most preferable one for pulverization of the rubber blocks. In addition, the volatile components produced by heating the rubber blocks as previously mentioned are combustible and they can be used as an effective heat source by burning them. The combustion gas produced by said burning may be employed as gaseous streams in the dry distillation, that is, pulverization and pyrolysis in the fluidized bed-forming oven according to this invention.
The rubber powder so produced is subjected to main dry distillation while causing it to be floated and fluidized in the oven. There have heretofore been proposed several methods for pyrolyzing rubber material such as used tires, and these methods are each a pyrolyzing method using a closed, fixed bed-forming oven. The practice of these methods will raise such problems that rubber material is difficult to feed continuously, the temperature of the rubber material is difficult to adjust because of a temperature gradient being produced between the inner and outer portions of the rubber material, and the time for the decomposition reaction is long.
On the other hand, the employment of a method for oxidation fluidized bed dry distillation (that is, fluidized bed dry distillation in the stream of an oxygen-containing gas) according to this invention, will be advantageous in that rubber material is allowed to be continuously fed, a fluidized bed is formed of rubber powder having a particle size of not larger than 5 mm, a temperature control is very easy since the rubber powder is mixed with a thermal medium, and a decomposition reaction is completed in a very short time since the fluidized bed formed by the oxygen-containing gas is partially burnt thereby to give heat to the system. In the fluidized bed pyrolysis according to this invention, the temperature control is important since it has an effect on the yield of oily components and carbonized components which are products obtained by the pyrolysis. The rubber material when heated, will decompose with some exothermic heat evolved therefrom, and the temperature control of the system can be made easy by controlling the amount of the exothermic heat evolved. More particularly, the temperature in the oven may be controlled or adjusted very precisely, for instance, by controlling the amount of rubber material fed while selectively using air only or an inert gas-diluted air, that is, adjusting the content of oxygen thereof introduced to form the fluidized bed. The process of this invention is economically advantageous in that it needs enough externally supplied heat to commence the decomposing reaction, after which it can maintain the decomposing reaction without such externally supplied heat since the heat necessary for said reaction is obtained by the combustion of part of the rubber material fed. The inside of the oven for fluidized bed pyrolysis is kept at temperatures of, preferably, 400.degree.-500.degree. C.
In the practice of the process of this invention, it is important to adjust the temperature for dry distilling the pulverized rubber to a certain fixed temperature within the range of from 400.degree. to 500.degree. C. in order to recover oily materials, particularly dipentenes, in a high yield.
The fluidized bed pyrolyzing oven which may be used in the practice of this invention is preferably provided with an agitator depending upon the shape of rubber material used. Such an agitator-provided oven is preferably used particularly when rubber powder produced is relatively large in particle size or is partially not uniform in particle size.
In practicing fluidized bed pyrolysis, the thermal media are not necessarily needed in cases where the rubber particles produced are in the finely divided form or are uniform in shape or particle size. Particularly when tire scrap is used as the starting material in the process of this invention, the carbon black incorporated in the scrap will be liberated therefrom and thereby function as a very suitable thermal medium for the fluidized bed. Carbon particles produced by the carbonization of the rubber material also function as a thermal medium. If thermal media are needed in the pyrolyzing step, they may be the same as used in the step of pulverizing the rubber blocks. When the rubber material is continuously fed to the fluidized bed-forming oven which is continuously operated, solid carbonized materials from the rubber materials will be accumulated as the pyrolysis and carbonization thereof proceed in the oven. Thus the carbonized materials so produced are allowed to overflow through an overflow pipe leading from the middle part of the oven, for their separation from the system. Even in cases where the thermal medium is used as mentioned above, a small amount of the thermal medium for the fluidized bed needs to be supplied from an external source of the thermal medium only at the beginning of operation of the oven, after which the system is self-supplied with such thermal medium produced as a by-product by the pyrolysis of the rubber material fed. From the view-point of effective use of solid carbonized materials produced by pyrolyzing rubber material, it is desirable that residual carbonized materials, carbon black and/or the like previously obtained pyrolysis be employed as such thermal medium being initially used as mentioned above.
The oxygen-containing gases which may be used in the fluidized bed-forming oven are not particularly limited, and include air and the mixtures thereof with an inert gas such as nitrogen, steam or carbonic acid gas or with a combustion gas. These gases may also be utilized as a heat source. The rubber materials which may be used herein are not particularly limited, and examples thereof are the vulcanizates and unvulcanizates of natural rubber and various synthetic rubbers.
In one aspect of this invention, in a single fluidized bed type heating oven, the rubber blocks are mixed with the melt adhesion-preventing solid particles, subjected to pulverization at 250.degree.-400.degree. C. to produce therefrom rubber powder sufficiently fine to form a fluidized bed thereof and then raised in temperature at 400.degree.-500.degree. C. while being kept in the form of fluidized bed, thereby to effect the thermal oxidation and pyrolysis of the rubber powder in the bed. In this case, the process of this invention comprises two steps of pulverization and pyrolysis, which steps are alternately repeated in the single oven. If there is used a two-oven system comprising an oven for the pulverization and an oven for the pyrolysis connected to each other in series, the two steps may be effected continuously. In this two-oven system, rubber powder approximately uniform in particle size produced in the pulverizing oven may be passed or supplied to the pyrolyzing oven either at the bottom thereof through a screw conveyor or at the top thereof through a hopper or the like. The method of supply in this case is determined depending on the kind and shape of rubber supplied.
It is to be understood that in the step of pulverization the pulverization of rubber blocks is principally effected while the pyrolysis thereof is simultaneously accessorily effected, and that in the step of pyrolysis the pyrolysis, of the rubber blocks so pulverized is principally effected while slightly additional pulverization is simultaneously accessorily affected. Thus it is to be also understood that the dry distillation of the rubber material is effected slightly in the step of pulverization and vigorously in the step of pyrolysis.
The oily materials obtained by the dry distillation of the rubber blocks are passed from the oven at the top thereof to a cooler through a cyclone. Since the oven according to this invention is capable of forming a fluidized bed therein and adjusting the temperature of the bed precisely by controllably varying the amount of rubber powder burnt, it is possible according to the process of this invention to produce oily materials in a substantially constant composition in an increased yield. Since the oily materials greatly depend for their yield on the cooling efficiency of a cooling device used, the device should be such that the oily materials are recovered in a high yield. For example, the oily materials are passed through an oil layer to effect a heat exchange therebetween thereby increasing the yield of the former. More particularly, the oily materials in a gaseous state is firstly cooled by the air into a condensate which is further cooled by water and then passed through the oil layer thereby completing the cooling thereof.
In the above-mentioned manner, the oily materials and the solid carbonized materials can be recovered in a total yield of about 95% based on the weight of the powdered rubber used. The remaining approximately 5%, which comprises mainly lower hydrocarbons such as methane and ethane, is dissipated or lost; if collected, however, it can be used as a gaseous fuel.
The oily materials and the carbonized materials are recovered in a total yield of about 90% based on the weight of the rubber blocks used.
In another aspect of this invention, in a single fluidized bed-forming oven provided with an agitator, used rubber materials such as used tires, are heated to 350.degree.-800.degree. C., preferably 400.degree.-600.degree. C., under agitation while producing carbon particles as a dry distillation residue by dry distilling said rubber materials, the carbon particles serving as a powdery thermal medium, to pulverize the rubber material and substantially simultaneously pyrolyze the thus-obtained rubber powder while forming a fluidized bed thereof in the stream of an oxygen-containing gas and burning part of the rubber materials. If the used rubber materials are exposed to temperatures such as 350.degree.-450.degree. C., they will be pulverized by collision and friction caused between the rubber materials and the vanes of the agitator, between the rubber materials themselves and between the rubber materials and the carbon particles as the dry distillation residue, thereby enabling the rubber materials to be dry distilled while being fluidized irrespective of their initial shape. This can be confirmed by a hammer test as follows. A glass cylinder is charged with some amount of sand and heated air is introduced thereinto at the lower end thereof in order to form a fluidized bed of the sand. In the thus-formed fluidized bed kept at 350.degree.-450.degree. C., wire-supported tire scraps are inserted for 10-60 minutes, after which the scraps are withdrawn from the cylinder and subjected to the hammer test with the result that the scraps are pulverized extremely easily by hammering slightly. In this aspect of this invention, the carbon produced as a by-product by dry distilling and decomposing the rubber materials as the feed in the system is used as the melt adhesion-preventing agent and, therefore, no such agent is required to be added to the system from outside the system. The preventing agent, however, may of course be incorporated with a thermal medium such as sand for use as such. When the process is continued for a long time such carbon will be accumulated in the oven; however, an excess thereof may overflow outside the system through an overflow pipe as previously mentioned. In view of the fact that the slices of unvulcanized rubber when dry distilled, will not form a fluidized bed thereof even under agitation without the use of melt adhesion-preventing agent such as sand, the above embodiment of this invention will be understood to be useful even in the treatment of such unvulcanized rubber. Also in this embodiment, the burning of the part of the fluidized bed serves to allow the decomposing or pyrolyzing reaction to proceed very rapidly, facilitates the control of temperature of the bed and enhances the yield of the oily decomposition products. For example, when dry distilling used rubber tires at 450.degree. C. they will give oily products in a yield of not less than about 50% by weight thereof. The decomposition products vary in composition with the decomposing temperature used. The decomposition products obtained by the dry distillation at 450.degree. C., when fractionally distilled, will be found to contain fractions of 150.degree.-190.degree. C. and 360.degree.-479.degree. C. in large amounts, that is, 70-80% by weight of the total of the oily products obtained. It is to be also noted that the oily products obtained are relatively uniform in composition. In contrast, the use of heated nitrogen or other inert gases supplied from outside the system as a heat source will make it difficult to secure a uniform temperature distribution in the fluidized bed and will result in a decreased yield and non-uniform composition of the oily products. The amount of heat obtained by burning part of the bed can be controlled and the temperature within the oven can therefore be precisely controlled by adjusting the amount of the used rubber tires fed while adjusting the amount of oxygen supplied by using air or an inert gas-diluted air as the gas for the formation of the fluidized bed. It is very advantageous from an economical point of view that the process requires the necessary heat from outside the system only at the initial stage of operation thereof and, after the start of decomposing reaction, it is self-supplied with the necessary heat by burning a portion of the rubber tire feed sufficient to supply said necessary heat to the system.